In numerous such aircraft, in particular for the purpose of ensuring that the aircraft can be used for multiple purposes in spite of the constraints set out below, the following are guaranteed:                ground clearance should be reduced as much as possible;        safety should be acceptable in the event of an emergency or crash landing;        aerodynamic resistance should be low; and        the aircraft should have excellent stability on the ground.        
These constraints lead in particular to providing landing gear mounted on the aircraft so as to be retractable, with the landing gear also operating in a specific manner in the event of an emergency, and in particular of a crash.
Furthermore, in certain aircraft, it is necessary for the landing gear to be compatible with naval requirements, i.e. for landing on a ship. In such circumstances, the landing gear must be steerable through 180° in either direction (±180°), it must be capable of being locked relative to its steering axis, and it must avoid a parasitic wobbling phenomenon known as “shimmy”.
In the field of rotary wing aircraft, the term “shimmy” designates rapid oscillation of a wheel about a pivot axis leading to strong interfering forces that can even lead to the undercarriage being destroyed.
In addition, the arrangement of a nose undercarriage must not impede the operation and the installation on the aircraft of detection equipment such as radars.
The fuselage of such an aircraft, or at least some of its structural elements, must also be compatible with the above-mentioned constraints.
Firstly, this is often incompatible with the occupants of the aircraft having good visibility between the passenger compartment and the cockpit.
Secondly, it is useful for the inside of the cabin to present an arrangement that allows its occupants to move without difficulty between the passenger compartment and the cockpit.
It is also desirable for the cabin to present an arrangement on its inside that leaves space available either for elements forming part of the aircraft, or for objects that are needed by the crew or the passengers.
However, in the past, the structures of fuselages and undercarriages have not enabled the desired results to be achieved.
At least, at present, complying with certain specifications is contradictory to achieving other functions, thus making it necessary to find compromises that are not always acceptable in practice.
With reference to the structures of rotary wing aircraft, mention is made below of a few documents of interest.
European patent No. 1 052 169 describes, in a helicopter, a truss of beams rigidly secured to the floor and to the skin at the bottom of the fuselage.
European patent No. 1 426 289 describes a helicopter structure that withstands impacts, and an energy absorber.
That structure forms a frame constituted by tubes of fiber-reinforced composite material, instead of I-section beams. The tubes are filled with foam material. The tubes are placed under the side wall at points where loads are induced in the event of a crash.
That document does not provide for a box having solid walls to be interposed at a distance from the side walls of the fuselage, above the auxiliary nose landing gear which itself disposed close to the center of the aircraft in its transverse direction.
French patent No. 2 629 045 describes a structural assembly for a light airplane. A passenger compartment is constituted by a central or main partition at the back of the compartment, by a front partition at the front end of the compartment, and by a hollow central beam extending in the longitudinal direction of the airplane.
That assembly is reinforced by the fuselage walls. On the rear partition, there are secured the rear portion of the fuselage and a main landing gear.
The front portion of the central beam is then used to receive a portion of the landing gear in the retracted position.
French patent No. 2 693 976 describes a helicopter fuselage having a central structure and, connected thereto: a front structure; a rear structure; and a landing gear.
Those structures support a transmission unit, a main rotor, and an engine.
The central structure has a framework provided with covering elements that define the outside shape of the fuselage. The framework is substantially in the form of a hexahedron with framework panels being assembled to one another.
British patent No. 724 999 describes a frame for an aircraft such as a helicopter. The frame forms a truss of tubes leaving large side openings. In order to form the fuselage, a skin covers the truss of tubes and a floor is placed on the bottom portion of the truss.
U.S. Pat. No. 4,593,870 describes a fuselage structure for an aircraft, in particular a helicopter. Under the floor of the passenger cabin, behind the cockpit, that structure has a truss of beams made of composite material. The truss makes it possible to obtain increased and progressive resistance in the event of a crash landing.
Transverse walls with central openings separate the cockpit at the front from the passenger cabin behind it.
U.S. Pat. No. 5,451,015 describes an aircraft fuel tank, specifically for a helicopter. The tank is reinforced so as to withstand an emergency landing, and it is located behind a solid transverse partition.
That partition, which is placed behind seats, is provided with ribs so as to contribute to withstanding loads during normal operation of the aircraft.
International patent WO 00/05130 describes a helicopter fuselage. That fuselage has a central portion with a front portion and a rear portion connected thereto.
The central portion is provided with means for transmitting forces due to the main transmission unit, to the main rotor, to the landing gear, and to the tail boom.
That document does not describe a fuselage structure having a cavity for housing landing equipment, nor does it describe a fuselage suitable for providing passengers and crew members with free passage in a longitudinal direction on either side of the fuselage.
With reference to undercarriages, mention is made below of a few documents of interest in this technical field.
French patent No. 2 608 242 describes a shock absorber for the landing gear of a rotary wing aircraft.
That shock absorber is for main landing gear including a rocker arm. It is then disposed in a position that is substantially vertical, and as a result its shock-absorbing function, i.e. resilient and damped absorption of the energy of the downward movement of the aircraft while landing and touching the ground, is performed in compression under load.
French patent No. 2 635 498 describes a device for steering the wheels of an aircraft nose undercarriage. A rod slides along its axis in a tube with its free end carrying the wheels.
A scissors connects the tube and the rod in pivoting. A retraction actuator includes means for limiting forces in the event of a crash. A shock absorber presents a longitudinal axis that coincides with the wheel axis.
French patent No. 2 647 170 describes a device for reducing the flexibility of a shock absorber for a helicopter undercarriage. A shock absorber cylinder includes a shock absorber piston and a shock absorber rod mounted to slide in leaktight manner relative to the cylinder and defining within the shock absorber a compression chamber containing a hydraulic fluid that is substantially incompressible.
An expansion chamber contains a hydraulic fluid adjacent to a compressible fluid under pressure, and communicates with the compression chamber via means for throttling the fluid expelled from the compression chamber.
French patent No. 2 677 951 describes an electrical steering device for an aircraft undercarriage. An electric motor is secured to a shock absorber box and is associated with a drive shaft which is disposed parallel to the axis of the box. A steering rod is secured to the drive shaft and to a pivoting tube.
French patent No. 2 689 088 describes a shock-absorbing actuator for a helicopter. It includes a function of limiting forces in the event of a crash. It comprises a strut and performs the functions of maneuver, absorbing shocks, and peak-limiting forces.
French patent No. 2 684 957 describes a peak-limiting device for a shock absorber for a helicopter landing gear. That shock absorber is hinged via ball joints. The landing gear is neither steerable nor retractable, and the peak-limiting device is always integrated in the shock absorber.
British patent No. 527 994 describes a device for steering an aircraft undercarriage. The steering axis of the wheel intersects its axis of revolution.
A hinge allows a rocker arm to rock relative to the structure of the aircraft. That device comprises two arms extending in elevation on either side of a sleeve for receiving the steering axis of the wheel, in order to position the height of said steering axis, i.e. position it in elevation.
U.S. Pat. No. 2,493,649 describes a drive system for steerable front wheels of an aircraft with a shock absorber that presents a longitudinal axis intersecting the axis of the wheel.
U.S. Pat. No. 5,944,283 describes a shock absorber for an anti-crash undercarriage. Its rocker arm axis is offset.
U.S. Pat. No. 6,257,521 describes an aircraft tail wheel, in which the axis of its rocker arm is offset in order to avoid interfering movements.
The teaching of those documents in particular does not make it possible to obtain a nose landing gear and/or a structure for the fuselage of a rotary wing aircraft for ensuring the following features simultaneously:                low ground clearance;        good safety in the event of a crash;        minimum aerodynamic resistance and instability;        landing gear that is unaffected by the instability phenomenon known as “shimmy”;        landing gear that is compatible with naval requirements;        a structure of an arrangement that does not interfere with radars or other detectors;        the occupants have good visibility towards the front;        it is easy for occupants to move about in the aircraft; and        inside spaces are made available.        